The invention relates to a sample injector for a liquid chromatograph. Such injectors are used for injecting the sample to be chromatographically separated into the chromatographic column.
Know injection systems typically comprise an injection loop which can be connected to the chromatographic system, i.e., the solvent delivery system and the chromatographic column, by one or several valves, and through which solvent flows at high pressure when no sample is injected. When sample is to be injected, the injection loop is uncoupled from the chromatographic system, one end of the loop is connected to a low-pressure metering syringe, and the other end is connected to a sample-intake arrangement which is moved such that it dips into a sample container. Then, the low pressure metering syringe is operated such that a desired amount of sample is drawn from the sample container into the injection loop. Once the sample is drawn in, the injection loop is switched back into the chromatographic system by means of the above-mentioned valves. The stream of solvent from the solvent delivery system then compresses the sample and transports it to the column.
With such known injection systems, air bubbles may arise in the injection system by small leakages or by degassing of the liquid in the injection system. Such air bubbles distort the separation process and therefore have a negative influence on the accuracy of measurements of sample components emerging from the column. Furthermore, if the original solvent is followed by a second solvent that turns out to be incompatible, e.g., mixing, reaction with the sample, this may lead to a loss of the sample or to variations in reproducibility. Such problems can be reduced by providing additional flushing circuits, but this increases the complexity of the injection system, leading to increased cost and decreased reliability. Furthermore, the addition of flushing circuits increases the dead volume leading to a deterioration of the chromatographic measuring accuracy.
An injection system having such an additional flushing circuit is known from the article "Automatic Liquid Chromatograph Injection and Sampling", Hewlett-Packard Journal, April 1984, pages 21-24. This known injection system comprises an additional flushing valve which permits flushing of the metering syringe and associated capillaries before each new injection of sample.
Relative to this prior art, it is an object of the invention to provide a sample injector which has a simpler design and which permits performance of repetitive injections in a simpler manner without suffering from the above-mentioned problems of known injectors.